This is the Wista Groundglass/Fresnel Sandwich combination used by Wista on their 8x10 field camera, and is offered here as a replacement for a lost or damaged groundglass/fresnel.
Wista employs this unique, fused combination of traditional groundglass with fresnel lens overlay, along with a protective glass closer to the photographer in the optical path. This 'sandwich' is known for especially good brightness with a standard ~300mm lens for an 8x10 camera.
This viewing instrument may work with other 8x10 cameras, but is only specified for Wista's 8x10.
Fused pairing of normal ground glass and fresnel lens